By the early 1970's, the copper foil industry had universally adopted a methodology of producing treated copper foil which separated the manufacture of the so called "raw" copper foil (electrodeposited on rotating drum cathodes) and the electrodeposition on such copper foil of a bond-enhancing "treatment", usually electrodeposited in multiple treater tanks or treaters. These distinctly separate manufacturing steps are now established within the world-wide copper foil industry and are enshrined within ever increasingly sophisticated equipment and process engineering. The output of such machines used in the printed circuit board (PCB) industry has also increased per individual unit, as the electric current applied to cathodes used in raw foil manufacture has more than doubled, and the increased output has driven the development of faster and faster running treaters. Simultaneously with this evolution the typical thickness of copper foil used in the industry has been reduced and continues to reduce.
These industry wide developments have created a situation where installed capacity is focused on larger and larger manufacturing plants which are increasingly inflexible in the range of products they can produce.
Because of the increased output per unit, copper foil production machinery is usually set up to run continuously on one grade of product. Any changes required to produce different products are costly in down time and are avoided where possible. It is also the case that high speed treaters need large rolls of copper from the drums to take advantage of their throughput potential. Rolls in excess of three thousand pounds are commonplace, and for such rolls of a typical substance weight of one ounce per square foot and 55 inches wide, it takes about 8.7 hours to provide the raw foil with the treatment in a treating machine at 20 ft/min, but only about 2.2 hours at 80 ft/min.
Traditional methods of providing copper foil with a bond-enhancing treatment consist of a sequentially applied plurality of electrodeposited layers. Usually, the first layer is electrodeposited on the matte side (side facing the electrolyte) of a roll of raw foil, such electrodeposit creating a dendritic layer whose role is to increase the true surface area, which in turn enhances the foil's "bondability" to typical polymeric base materials. The dendritic layer, is usually followed by a "gilding" layer of pure copper metal, then a "barrier" layer, and finally a stain proof layer. The details of such processes are disclosed, by way of example, in U.S. Pat. Nos. 3,857,681 (Yates, et al.), and 3,918,926 (Wolski, et al.).
Generally, it can be said that first two steps of the treatment (usually executed in four consecutive plating stages) change the micro-topography of the matte side of the foil, while the remaining steps, barrier layer and stain proof layer, change the chemistry of the surface by virtue of the application of micro layers that render the properties of the bonding treatment durable and resistant to various forms of corrosion. The bonding treatment operation is conducted in machines, called "treaters", wherein rolls of raw foil are unrolled in a continuous manner and fed into the treater by means of driven rollers (similar to the way in which a web of paper is handled in a printing machine), rendered cathodic by means of cathodic contact rollers and passed in a serpentine fashion through a plurality of plating tanks, facing, in each tank, a rectangular anode. Each tank has its own supply of appropriate electrolyte and its D.C. power source. Between the tanks, the foil is thoroughly rinsed on both sides. The purpose of this operation is to electrodeposit on at least one side of the foil, usually the matte side, microprojections of complex shape, which ensure that the foil will be firmly anchored to the base polymeric materials used in fabricating the copper clad laminates.
In order to handle a very thin copper foil in the manner described above, treater machines must be heavily engineered. They are equipped with sophisticated web tension control devices that must maintain uniform tension through the varying temperatures in the plating tanks which expand and contract the foil. Such control systems match roll speeds to the varying length of the web and are essential to avoid slippage and creasing of the foil. All path rolls must be precisely machined, particularly the "contact rolls" which pass electric current into the copper foil. Consequently, treater machines, built by specialized engineering companies have become increasingly expensive and can cost in excess of $10 million per treater.
Because of the inherently high investment cost of such treaters, they are designed to operate at high speeds, for example up to 150 ft per minute. In contrast, the speed of the raw foil produced on the rotating drum machine is much slower, and can be about 5 ft/min. for one ounce foil (such speed being dependent on the applied current and the drum diameter and width).
In a typical foil manufacturing plant, therefore, one treater has to "serve" the output of many drum machines. Those skilled in the art have thought that increasing the speed on such equipment can only be done by balancing the requirements of linear speed with commensurately adjusted process parameters. To achieve any defined character of electrodeposit on the copper foil obviously requires that a certain current density to be applied for a certain time in any given bath if the anode size remains constant. Increasing web speed on a treater clearly reduces the "plating time" which is the amount of time a point on the foil is exposed to the anode. Similarly, any compensating increase in applied current will change the nature of the crystals formed at the point of deposition because of the increase in current density. Plating time can be increased by having deeper tanks and longer anodes, but the stability of the web as it passes through the individual plating tanks is threatened once the unsupported length of the web on the down pass is greater than the width supported by the path rollers. Controlling these factors adds enormously to the cost of the modem breed of treating machines.
The present inventions substantially avoids the problems outlined above and provides a uniquely novel method for optimizing the desired treating conditions in a way hitherto impossible by conventional means.
To explain how the invention makes these improvements possible first requires an explanation of how the objectives of copper foil treatments are presently achieved and how they are improved by the present invention.
The first treatment layer is a dendritic deposit whose role is to increase the foil's true surface area and thus promote good "bondability".
In its travel through the dendritic deposit process tank, foil typically passes in front of one or two rectangular anodes disposed parallel to the foil path and each being of a length calculated to provide a dwell time appropriate to the speed of the web and the plating time required. The actual deposition time for this stage of the treatment may be only between four or eight seconds. In order to deposit a mass of treatment that offers bondability, within such a short deposition time, it is customary to use very high current densities, corresponding to the limiting current density of the treatment process. Moreover, the limiting current density assures formation of dendritic deposits that are good for bondability.
Such an approach to the bonding treatment process is not only expensive (cost of the treaters, losses of production due to the "handling" of delicate foil by long and complex treater machines which can easily cause wrinkles, scratches, etc.), but also produces a treatment structure that is "high profile". This is because the high-speed treater line, combined with high current density, creates mass transfer conditions at the foil-electrolyte interface which in turn leads to the distribution of treatment over the micro-profile of the base (raw) foil in a manner which favors the peaks and neglects the valleys. Peaks are thus "overcrowded" with the bonding treatment at the expense of the valleys. This is an undesirable condition of so-called poor micro-throwing power and can, therefore, be responsible for degraded dielectric properties in PCBs made with such material.
Treatment with overly high micro-projections concentrated on the peaks of base foil is a poor raw material for fabrication of printed circuits. The cross-section of the foil is chain-saw like, with "teeth" that penetrate very deeply into polymeric substrates. Consequently, this increases the time necessary to etch away unwanted copper, the particles of copper tend to remain deeply embedded in the resin, affecting unfavorably dielectric properties of printed circuit boards and diminishing layer to layer dielectric thickness in the fabrication of multi-layer printed circuit boards.
We have determined that a reverse of the above-described condition, a good micro-throwing power, is desirable, in the electrodeposition of bonding treatment. This can be achieved if the treatment's microprojections, instead of overcrowding micropeaks, descend deeper toward the micro-valleys. Good bonding ability is achieved not by excessive height of the treatment at the peak, but by better distribution of the individual treatment particles (microprojections). If the height of microprojections is decreased, but their number increases, the bonding ability of the foil will remain the same, but such foil will be endowed with more desirable characteristics, namely low profile.
We have found that the shortcomings of the existing treatment process can be improved if the micro-roughening steps of the treatment are conducted under the conditions of low current density, long deposition time, and in the presence of a specific addition agent, so as to achieve uniform distribution of the treatment layer on the surface of the new foil.
Surprisingly, under such conditions, i.e., where conditions of the mass transfer are drastically different from the mass transfer occurring in the traditional treatment process, the need for a dendritic or powdery stage of the treatment followed by the gilding layer stage, is eliminated.
The one step micro-roughening treatment we have discovered, especially when electrodeposited on the matte side of the base foil, has a structure which enhances the true surface area and thus promotes good "bondability". The treatment particles are also quite hard and mechanically resilient. Furthermore, the treatment particles are evenly distributed over the micro-profile of the base foil, that is, in the valleys as well as on the peaks. This means that the treatment particles are no longer peak orientated, but are much more uniformly distributed on the slopes and in the valleys. Thus the treatment combines the desideratum of lower profile with excellent bondability.
This simplified one-station treatment can only practically be applied at the slow speeds that are a feature of the present invention.
In our simplified foil machine, the raw foil emerging from the rotating drum section is guided into and through the first treating tank (station) with the micro-roughening electrolyte and passed before a pair of rectangular anodes (each 20 to 40 inches long). The electrodeposition of the treatment is delivered with a current density of from about 40 up to 100 A/ft.sup.2.
After completion of the micro-roughening step, the foil can be guided directly through a sequence of a barrier layer applying station (U.S. Pat. No. 3,857,681), a stainproofing station (U.S. Pat. No. 5,447,619) and a spray of chemical adhesion promoter.
Only in the present invention is it practical to adopt the improved technology described above, as it is only at the web speeds associated with raw foil production that there is sufficient time to operate a low current density micro-roughening process that produces a low profile, high density deposit as described above. Also, since the foil path in one-step drum-treater machine is much shorter than in a traditional treater machine, production losses are minimized, while the cost of one-step machine is only a fraction of that of traditional treater machines.
A further advantage of the present simplified foil production unit is that it eliminates the need for unwinding and winding the rolls of foil two times over, thus further reducing production losses. It also allows short runs of specialty products and can be equipped with in-line sheeting capability.